feat: add provider-backed rate limiting boundary

[ZhiXiao-Lin]

Summary

• expose a public async RateLimitProvider contract for atomic shared-budget acquisition
• keep the existing process-local behavior through InMemoryRateLimitProvider
• scope SHA-256 subject digests by stable policy ID so plaintext header values and bearer credentials do not cross the provider boundary
• reject provider errors, invalid policies, and conflicting settings without bypassing the guard
• document the provider boundary and its intentionally limited scope

Scope

This delivers the provider-backed distributed rate-limiting boundary requested by the Cloud C0 work. It does not select a distributed backend or implement the remaining streaming cancellation, backpressure, authorization metadata, graceful-drain, or telemetry work.

Progresses #1

Validation

• cargo fmt --all -- --check
• cargo test --features security --test security
• cargo test --features security --lib security::rate_limit
• cargo test --all-features --locked
• cargo clippy --all-targets --all-features --locked -- -D warnings
• git diff --check